Electrostatic printing



ELECTROSTATIC PRINTING Harold G. Greig, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed July 31, 1956, Ser. No. 601,221

13 Claims. (Cl. 96-1) Ihis invention relates to electrostatic printing and particularly, but not necessarily exclusively, to improved electrophotographic recording elements for electrostatic printing and to improved methods of electrostatic print-A ing utilizing said improved recording elements.

An electrostatic printing process is that type of process `for producing a visible record, reproduction, or copy which includes, as an intermediate step, converting a light image or electric signal into an electrostatic charge pattern on an electrically-insulating base. The process includes the conversion of the electrostatic charge pattern into a visible image by the application thereto of electrostatically-attractable particles. An electrostatic printing process using a photoconducting insulating stratum vto convert a light image'into an electrostatic charge pattern v is described by C. I. Young and H. G. Greig in Electro- `fax Direct Electrophotographic Printing on Paper, RCA Review, volume XV, No. 4, pages 469 to 484 (December 1954), hereinafter referred to as `the Young and Greig publication. y

A typical electrostatic printing process utilizing a photoconducting insulating stratum includes first producing a blanket electrostatic charge on the surface of the photoconducting stratum. The electrostatic charge Vmay be stored on the surface -for a time in the dark. The rate at which this stored electric charge is dissipated when the stratum is maintained in darkness is hereinafter referred to as the dark decay of the stratum. Withinv the period in which a substantial charge remains, a light image is focused on the charged surface, discharging the portions of the surface irradiated with light, leaving the remainder of the surface in a charged condition, and thereby forming an electrostatic image thereon. The electrostatic image is rendered vvisible by applying to the electrostatic image a developer substance, such as a pigmented thermoplastic resin powder, which, is held electrostatically to the charged areas of the surface. The visible image thus formed may be `fixed directly to the surface, for example, by'fusing the image thereto.

One type of recording element usable in the foregoing electrostatic printing process comprises a cellulosic sheet,

coated with a particulate photoconductor dispersed in an electrically-insulating, tilm-forming'vehicle, for exam ple, ordinary paper coated with particulate photoconducting white zinc oxide dispersed in a silicone resin.l Such recording elements produce satisfactory prints if `they arestored at normal or Vabove normal humidity for a substantial period of time just prior to use.- However, poor prints result if the element is stored justprior to use in a below normal humidity. Generally, the lower the humidity the poorer the print. It is beleved that the reason for such poor results isgthat the paper bacling becomes non-uniformly more insulating at lower humidi` ties. in such condition, the electric field resulting from Y the blanket' electrostatic charge appears :non-uniformly` across the backing and the photoconducting stratum instead of uniformly across only'vr the photoconducting stratum. This greater distance,` and'non-uniforn fieldY w Patented A121511 1961 ard web coating machines, (3) the backing is not com' patible with the photoconducting coating, (4)l the backing is too rigid or inflexible for many uses, and (5) the color of the backing is objectionable.

An object of this invention is to provide'improved electrophotographic recording elements for electrostatic'Y printing and improved methods of electrostatic'printing including using the improved recording elements of the invention. I

Another object is to provide improved recording elements for electrostatic printing which Vrecording elements, are cheap, flexible and easily preparedin standardweb coating machines.

A further object is to provide improved electrophotographicv recording elements for electrostatic printing whose printing characteristics are Substantially independent ofthe humidity invwhich it is stored prior to use.

In general, the recording elements of the invention comprise a photoconducting stratum contiguousV with al substratum comprising a particulate semiconductor di's persed in a binder, said substratum having a short dark decay relative to the dark decay of said 'photoconducting stratum. By placing a substratum of the type described contiguous with` the photoconducting stratum, the photoconducting stratum is'relatively independent of the characteristics of the backing during charging and exposure. It is believed that at low humidities as well as high hu-A midities, the electric iield at the time of charging appears across the backing, stratum and substratum. But, because of the relatively short decay of the substratum, a relaxation occurs in the substratum Strengthening the ei'ective electric field across the photoconducting stratum. Also, improved lateral conductivity of the substratum provides more uniform effective electric elds lacross the photoconducting stratum. Y

Anexample of a recording element of the invention comprises a paper sheet having coated thereon a'substratum including a particulate semiconducting zinc-oxide dispersed in a cellulose acetate resin and, coated thereon, a photoconducting stratum having a long dark decay-relative to the dark decay of said substratum and includingv `prising'the stratum and substratum' described` above both with or without a backing.

Theforegoing objectsand otherfadvantages are more fully described inthe following detailed description when read' inconjunction with the accompanying drawingv in which: n

M Figur'e'l is a sectional, elevational View of a typical recording V'element according tothe invention,

Figure 2 is a partially-schematic, sectional View ofan Vapparatus for producing. a blanket electrostatic charge upon a recording element produced bythe methodof the invention,v ,f

Figure 3 is apartially-sectional, elevational viWLof' jan apparatus for projecting a light j to form a contact image upon the charged'recording element-fof 'Figure 2,y

and

Figure 4 is a sectional view of an apparatus vfor developv ing Van electrostatic image upon the recording elementv produced in Figure 3,r andV *Figure S-is a `family vof curves illustrating tliedark de-V cays of combinations ,of various Zinc oxidesiand:variousV Yehcles) j a *i v in Y .Y Av2 Similar reference characters are applied to similar structures throughout the drawings.

A detailed example for preparing and using an electrophotographic recording element in accordance with the present invention will now be given.

- Example .L -Referring to Figure l, a recording elelment according to the invention is prepared as follows:

a rst mixture is prepared by dissolving 20 grams of soluble casein in 200 grams of water and l cc. ot 8% aqueous ammonium hydroxide. Then about 50 grams of semiconducting zinc oxide such as AZO Z22-33 marketed by the American Zinc Sales Company, 420 Lexington Avenue, New York, NY., is dispersed in the solution. This dispersion is coated on a paper backing 19 by any conventional coating process, and the coating dried thereby producing a substratum 2l on the paper backing 19. Y

A second coating mixture is prepared by mixing S grams of 60% solution of silicone resin in xylene, such as GE Sli-82 marketed by the General Electric Company, Silicone Products Division, Waterford, New York, 106 grams of toluene, and 120 grams of white photoconducting zinc oxide, such as Florence Green Seal-8 French Process Zinc, marketed by the New .lersey Zinc Company, Palmerton, Pa. The mixture is ballmilled to a smooth uniform consistency and is applied to the surface of the substraturn 21 and then dried thereby producing a photoconducting stratum 23 upon the semiconducting substratum 2l. Any standard coating technique may be used to produce the foregoing substratum 2l and stratum 23 such as owing, spraying, dipping, spin coating or brushing on.

Upon drying, the recording element is ready for use. It may be stored for long or short periods of time in a hot or cold storage wherein the humidity is high or low. Regardless of the humidity of the storage conditions, the recording element will produce good prints.

The recording elements of the invention may be used for electrostatic printing ,by any of the methods described in the Young and Greig publication, op. cit. By way of example and referring to Figure 2, the recording element of Example 1 may be utilized in an electrostatic printing process according to the following steps. The recording element is placed with the backing 19 upon a grounded metal plate 25 and in darkness, an electrostatic charging device 6l passed over the photoconducting stratum 23 to provide an electrostatic charge thereon. The charging device l. may comprise an array of tine wires 53 mounted near the grounded metal plate 25. A source of DC. voltage is connected between the wires 53 and the grounded plate 25 to provide a negative charge on the wires with respect to grounded plate 25. The voltage should be sulciently high to cause a corona discharge adjacent the wires. lt is preferred to applyl about 6000 volts with respect to ground to the wires 53 when charging the surface of the recording element ofthe example. The surface of thefrecording element passing under the charging device Si becomes charged negatively with the preferred voltage` applied. The apparatus and process may produce a blanket positive charge if the polarity of the wires 53 is positive with respect to the grounded plate 25.

v The Vnext step in the process is to discharge selected areas of the charged surfacevof the recording element in order to produce an electrostatic image thereon.

Referring to Figure ,3, this may be accomplished byV placing av photographic transparency 61 upon the charged surface 23 and exposing to light derived, for example,`

from a lamp 59 in the manner of conventional contact printing. An exposure of about l second from a 100 watt tungstenv lamp aboutl 24 inches fromthe recording element for a normal density photographic 'transparency has been found to beadequate. The light image may be produced by any of the ordinary photographic processes as by projection, contact, or reex. Any type of electromagnetic radiation may be used provided a portion falls within the spectral sensitivity of the photoconducting stratum 23. ln Example 1 visible blue and ultraviolet light may be used. For other photoconducting Strata, visible light, infra-red, or ultra-violet may be used.

Wherever the light strikes the surface of the photoconducting stratum 23, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern of charges corresponding to the nonillurninated areas of the light image.

The electrostatic image may be storedfor a time it desired. Ordinarily the next step is to develop the electrostatic image with a iinely-divided developer substance such as a finely-divided powder or an ink mist. Referring to Figure 4, development of the electrostatic image is preferably accomplished by maintaining the recording element in darkness and passing a developer brush 55 containing a developer powder across the surface of photoconducting stratum 23 bearing the electrostatic image. Areas 27 of` developer powder are deposited on those areas of the surface retaining an electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example powdered iron, and the developer powder. The mixture is secured in a magnetic eld by a magnet 57 to form a developer brush.

A preferred carrier material for the developer mix consists of alcoholized iron, that is, iron particles free from grease and other alcohol soluble impurities. These iron particles are preferably relatively small in size, being in their largest dimension about .002 to .008". Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat wider range of sizes from about .001 to .020".

A preferred developer powder may be prepared as follows: A mixture comprising 200 grams ofv 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its` homologs) marketed by the Pennsylvania Industrial Company, Clairton, Pa., 12 grams of Carbon Black G, marketed by the Eimer and Amend Co., New York, `N.Y., l2 grams ofv spirit Nigrosine S.S.B. marketed by the Allied Chemical and Dye Co., New York, NSY., and 8 grams of Iosol Black marketed by the Allied Chemical and Dye Co., New York, N.Y., are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short a time as possible.V The melt is poured into a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball-milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use a developer powder. This powder takes on a positive electrostatic Ycharge when `mixed with glass beads or iron powder. lt therefore develops an electrostatic image composed of negative charges. Two to four grams of the developer powder and 100 grams of the magnetic carrier material are blended togetherl giving the preferredy developer mix.

Other ratios may be used. y The-developer powder may be chosen from a large 'class of materials. The developer powder is preferably electrically-charged to aid in the 'development of the electrostatic latent image. The powder may be electrically-charged because the powder (l) is electroscropic, or (2) has interacted with other particles with'which it is triboelectrically active or (3) has beenV charged from an electric source such as a corona discharge. Examples ot suitable developer powders are powdered zinc, powdered copper, carbon, sulfur, natural and synthetic resins or mixltures thereof.r

The developer powder. may be applied to the electrostatic image in other ways, for example, it may be dusted on to the image, or it may be mixed ywithk glass beads or other suitable carrier particles, the mixture. then brought into .contact with the surface of the pliotoconducting stratum.V The beads serve merely as 'a temporary carrier,

vresin under releasing the powder particles `upon contact with 'the charged surface.

The type of developer powder described is a'positivelycharged powder and will adhere readily to negatively charged areas of the electrostatic image. In the developed image described, the developed areas of the image correspond to the non-illuminated portions of the light image. If the photoconducting stratum is charged positively, and the same steps are carried through as above described, a reverse image is obtained. If a negatively charged powder is used in place of the positively charged powder, then a reverse image is obtained in the iirst case and a positive image is obtained in the alternative case.

, The developed image 27 is now fixed to the photoconducting stratum 23. `If the developer powder or the.

vvehicle in the photoconducting stratum 23 has a relatively low melting point, the developed image maybe xed by heating, for example with an infrared lamp, to fuse the powder to the surface. Sulfur or synthetic thermoplastic resin powders may be fixed in this way. Alternatively, the developed image 27 may bepressed into the stratum 23. Another method of iixing the developedimage 27 is to apply a thin coating of a solvent for the material of the developed image 31. The solvent softens the developer powder particles and causes them to adhere to one another and to the photoconducting stratum 23. Alternatively, a solvent may be vused to soften the photoconducting stratum 23 and cause the developed image to adhere thereto. Upon standing, and preferably with the application of a slight amount of heat, the solvent is evaporated from the recording element.A

Prints made with the recording element of Example 1 which has been stored just prior to. use at normal or above normal humidity conditions by the foregoing printing procedure are comparable with those made on a similar recording element having no substratum 21. If the two recording elements are thoroughly ldried just prior to use, as by heating for five minutes at about 150 C., the recording element Without the substratum 21 blanks out entirely or prints only in patches, whereas the recording element of the invention still gives good prints.

Figure 5 illustrates the diiierence in dark `decays between the photoconducting stratum 23and the substream 21 of the recording element of Example l. The photoconducting coating composition (Florence Green Seal-8, toluene and silicone) is coated on aluminum foil and dried. The coating is then maintained in vdarkness at a constant controlled humidity for about 2 hours, then charged as set forth above. The surface voltage was measured continuously after charging. The curves 71 and 73 represent the surface voltage with respect to time for this coating at about 15% relative humidity and 907 relative humidity respectively at about 70 F.

The substratum coating composition (AZO ZZZ-33,

water and casein) was similarlycoated on aluminum foil, i

dried and charged. The curve '75 represents .the surface voltage with respect to time for this substratum coating at` about 5% relative humidity and about 70 F. *Above 50% relative humidty, no surface voltage is detected. Note that, although the charging is Vthe same, the charge is dissipated extremely rapidly inthe case of the substratum coating as shown by the curve75 compared with the photoconducting coating as shown by the curves 7l and 73. The combination of AZO X21-33 and silicone similar conditions is illustrated by the curve 77.

Example 2..-"Athin porous paper is impregnated with the first coating mixtureV O'PEXa'mple .l and dried.A YThe y second coating mixtureof Example 1 is then applied to Vone side of this impregnated sheet and dried. l'Ille substratumlwhich comprises the impregnated paper acts as a iloating ground'or a conductinglink to groundduring charging. Under dry .humidity conditionsftherecording,

elementris-not attracted to the .ground pjlate (does not flock devi/11) during charanga@ Will Print vin, humidity conditions where a recordingH element 'without a `semicon' ducting substratum completely blanks out.

Example 3.-A thin relatively porous paper sheet, such as a manifold bond, is iilled with a semiconducting zinc oxide powder. This can be done by rubbing Vor brushing a thin suspension or paste of the semiconducting zinc oxide into the interstices of the paper and then removing the vehicle by drying. The preferred method however, is to Vadd the zinc oxide to the pulp at the time the paper be prepared by a single coating'step. This recording'ele,

ment prints the same as the recording element in Example 2.

Examp'le 4 A paper sheet is coated with a substratuml coating mixture comprising 10 grams of cellulose acetate suchas Eastman Kodak No. 4644 Vis. 3, marketed by the Tennessee Eastman Corporation, Kingsport, Tennessee; 200 grams of acetone and grams of AZO`ZZZ-33 zinc oxide marketed by the American Zinc Sales Co., and dried, producing a substratum on the paper backing. The substratum is then coated with the second coating mix of Example 1 producing ra photoconducting stratum upon the substratum.

The recording elements of the examples utilize a semiconducting zinc oxide and a photoconducting zinc oxide. In the research work to developy an'electrophotographic recording element described in the Young and Greig pub-V lication, op. cit., it was noted that several of the zinc oxides tried for making recording elements, did not print is ideal for the'substratum of the present invention. These that produces ai layery that prints is useful in the photoanixture onfaluminurn `foil and dry.f'1`hen c ool to liquid `nitrogenj.ten'xl'aeraturesY (about f-.60y .-C.) and-xirradiate U 3 650. A. radiation, printable stratum exhibits .a lai/eases ,et Ofertas lsfansenca Awa-Printables? zinc oxides in any binder or as a filler ina base sheet itself provide a smooth support for the kphoteconducting stratum in thev electrostatic printing process. The recordingelements of the invention are relatively independent of moisture content in the supporting sheet orbacking and one may omit Vthe backing 'completely if they are suffi-y cientlfystrongf 4 The semicondueting zinc'oxides are identical in ap-VA 'pearance With the so-.called photoconducting zinc oxides.

I. Prepare a zinc oxide-vehicle mixas .described for the second coating mixture of Examplel. Coat the mixtureon laluminum foil and -dry. "Thenf attempt Vto producea print as described in Example 1. v A zinc oxide conducting stratum. A zinc oxide that does not print vor prints poorly s useful in the substratum.

" II. Prepare a zinc oxide-vehicle mixas described for the second coatinggmixture, of Example 1., Coat Ythe accenna poorly-printable stratum exhibits a green or yellow luminescence.

III. The zinc oxide sample is tired in hydrogen at about 1000 C. for about 10 minutes. The sample is cooled and irradiated with 3650 A. radiation. The Zinc oxides which produce printable strata luminesce brightly whereas the zinc oxides which produce non-printable or poorly printable strata luminesces weakly or not at all.

The table compiles the results of the three tests on typical commercially available zinc oxides and indicates the correlation between these tests. The zinc oxides marked V in column I may be used in the photoconducting stratum while the zinc oxides marked o may be used in the semiconducting substratum.

faint green. dead.

nearly dead.

Do. b 11130.

ri t rfreen. Spec. Pure: g a NJ. Zinc 4 Do. Spex Standard Do. Johnson-Matthey Do. Miscellaneous:

E. and A.1 Wet proc..-, Do. N..T.Z.4 Spec. No. 3 dull green. Florence Green Seal (staudard).-- v lavender bright green.

(e)=printability, v=good, x=loWer contrast, o=does not print.

(b)=luminescencc at liquid nitrogen temperature in silicone resin: 3650 A, exciting radiation.

(c) luminescence at room temperature under 365D A. lamp after tiring ai; 1000 C. in hydrogen.

1 Mallinckrodt Chemical Works, New York, N.Y.

2 Eagle Picher Sales Co., Philadelphia, Pa.

American Zinc Sales Co., New York, N .Y.

4 New Jersey Zinc Co., Palmerton, Pa.

Spex Industries, Queens Village 27, NX.

Johnson, Matthey and Co., 73 Hatton Garden, London, England. 14" Ilirver and Amend Co., Division of Fisher Scientific Co., New York While the reason for this improvement is not fully understood, the following explanation is offeredV to aid in understanding the improvement. In the recording elements of the invention, the substratum 21 has a very rapid dark decay and appears to act as a sink or oating ground which has enough lateral and transverse conductivity to permit developing adequate uniform electric field across the photoconducting stratum just prior to exposure. The photoconducting stratum has a very long decay with the result that the electric fields from the charging apparatus are more effectively utilized.

Other semiconducting materials which have shown the same effect when substituted for the semconducting zinc oxide above are:

Any of the foregoing materials maybe substituted for the semiconducting zinc oxides of the examples. It is Cata- Sales Sales Sales only necessary that the. material in combinationiwith a vehicle produce a substratum having a short dark decayA relative to the dark decay of the photoconductiug stratum.. For high speed printing, the thickness and composition of the substratum is preferably adjusted to possess the shortest dark decay possible.

The materials of the'foregoing Vparagraphs are operative when dispersed in a wide range of binders to oxide ratios as an undercoat or impregnation material. One may use small amounts of binder in ratios up to one part by weight of binder to 21/2 parts by weight of semiconducting zinc oxide. The upper limit is set by loose oxide particles interfering with the photoconduct-` ing coating, the lower limit is set by the ratio at which conductivity of the substratum is too low for the desired application. v

The requirements for the vehicle of thesubstratum are much less stringent than for the vehicles used in the photoconducting stratum. When the substratum is used as a film or coating on the cxible recording element, the substratum should of course, have the necessary flexibility, adhesions, shelf-life, desired in the composite sheet. If the substratum is to be overcoatedwith a photocon-v ducting stratum, it is desirable that the vehicle in the substratum have a different solvent solubility than that of the vehicle for the photoconducting stratum, so that the photoconducting stratum may be applied without un due mixing or dissolution in the two strata. Many of the desired properties of the substratum can be obtained by the use of additives or modifiers in the mix. For example, plasticizers may be added to gain flexibility, surface active agents may be added to gain adhesive characteristics, toughening agents may be added to gain strength. The vehicle for the substratum may be asynthetic or natural resin or wax and may be organic solvent or water soluble. Examples of suitable vehicles are cellulose acetate, cellulose acetate butyrate, polyvinyl acetate, polyvinyl chloride, polyvinyl chloride acetate, shellac, rosin, carnauba wax, beeswax, paran, polyethylene glycol, casein, hydroxyethyl cellulose, carboxymethyl cellulose or gum.

The photoconducting stratum 23 must be a material which exhibits a substantial change in electrical conductivity upon exposure to light, Vsuch that an electrostatic charge stored on the surface thereof may be discharged. The photoconducting stratum 23 determines the spectral response, the speed of response and the contrast characteristic of the recording element. By a proper choice of materials for the photoconducting stratum 23, any spectral response, speed of response or contrast characteristic overV a Vwide `range may be obtained. Many different powdered photoconductors may be used in the photoconducting stratum in place ofthe white zinc oxide. For example, one may use dye-sensitized white zinc oxide, cadmium-zinc sulfide, or panchromatically sensitive or buff zinc oxide, as described in U.S. Patents 2,727,807

' ductor.

and 2,727,808 to S. M. Thomsen. The photoconducting stratum may be any of the photoconducting coatings described in the Young and Greig publicatiomibid.

` The vehicle for the photoconducting stratum k23 is essential and may comprise any one of a number of substances which are film-forming and electrically-insulating. In place of a silicone resin, one may use any of the following vehicles: polystyrene, polyvinyl acetate, co.-

polymers of vinyl Vchloride-acetate, cellulose ethers, cellulose esters, methyl and ethyl cellulose, cellulose nitrate, shellac, paraflin'wax, and carnauba wax. The photoconductorm'ay be suspended in the vehicle in any one of several ways. The simplest Way is to dissolve the vehicle in an organic solvent capable of effecting solution and then mixing in the powdered. photocop- Alternatively, the photoconductor may be dry blended, as by kneadingwiththe vehicle heated.- to a Vsuiiicientlyrhigh temperature to render it plastic."

The proportion Aof photoconductor to vehicle/in the 9.' photoconducting stratum may vary over a very wide range. The preferred ranges are about 50%Ato 90% by weight of photoconductor to about 50% to 10% by weight of vehicle. The optimum proportion depends upon the nature of the photoconductor, the nature of the vehicle and the results that are desired.

The examples describe recording elements including a paper backing which provides a mechanical support for the stratum and substratum. The backing may be any web material but is preferably cellulosic such as ordinary paper or cellophane. are sutliciently strong, the backingmay be omitted.

There'have been described improved electrophotographic recording elements for electrostatic printing and improved methods of electrostatic printing utilizing the recording elements of the invention. The recording elements of the invention are cheap, easily prepared in standard web coating machines, exible and are substantially insensitive to the relative humidity in which they are stored prior to use in an electrostaticprinting process.

What .is claimed is:

1. An electrophotographic recording element including a photoconducting insulating stratum comprising particles of photoconducting zinc oxide dispersed in an electricallyinsulating, solid, `film-forming vehicle in contact with a semiconducting substratum comprising particles of apsemiconducting material selected from the group consisting of ksemiconducting zinc oxide, stannic oxide, titanium dioxide and basic lead carbonate, in a solid, film-forming vehicle, said photoconducting stratum having a long dark decay relative to the dark decay of said substratum, the weight ratio of photoconducting zinc oxide to vehicle in said stratum being between 50/50 and 90/ 10, and the vehicle being present in said substratum in proportions up to one part by weight vehicle to two and a half parts by weight semiconductingmaterial.

2. A recording medium according to claim 1 wherein said stratum and substratum are self-supporting.

3. A recording element according to claim 1 wherein said substratum is coated upon a support.

4. A recording element according to claim 1 wherein said substratum is impregnated into a porous support.

5. An electrophotographic recording element comprising a web of cellulosic material, a semiconducting substratum comprising particles of a semiconducting material selected from the group consisting of semiconducting zinc oxide, stannic oxide, titanium dioxide, and basic lead carbonate, dispersed in a ksolid film-forming v vehicle upon a surface of said web, anda photoconducting insulating stratum comprising particles of photoconducting-zinc oxide dispersed in an electrically-insulating, solid,

film-forming vehicle upon said substratum, said photoconductingv stratum having a long dark decay relative to the dark decay of said substratumthe weight ratio of photoconducting zinc oxide to vehicle in said stratum being between 50/50 and l90/10, andthe vehicle being present in said substratum in proportions up to one vpart by weight vehicle to two and a half parts by weight semiconducting material.

6. An electrophotographic recording element including If the stratum and substratumA #zinc oxide and between 10 and 50 weight parts vehicle,.

. 10 weight vehicle. to two and a half parts by weight semiconducting material.

7.'An electrophotographic recording element according to claim 6 wherein said semiconductor comprises zinc oxide.

8. An electrophotographic recording element comprising a paper support sheet, a semiconducting substratum including a particulate semiconducting zinc oxide dispersed in a cellulose acetate vehicle upon a surface of said support sheet and, upon said substratum, a photoconducting insulating stratum having Va long dark decay relativeto the dark decay of said substratum and cornprising a particulate photoconducting zinc oxide dispersed in a silicone resin vehicle, the weight ratio of photoconducting zinc oxide to vehicle in said stratum being between 50/50 and 90/ 10, and the vehicle being present in said substratum in proportions up to one part by weight vehicle to two and a half parts by weight semiconducting zinc oxide.

9. An electrophotographic recording element comprising a paper support sheet, a semiconducting substratum including a particulate semiconductingwzinc oxide dispersed in a casein vehicle upon a surface of said support sheet and, upon said substratum, a photoconducting insulating stratum having a long dark decay relative to the dark decay of said substratum comprising a particulate photoconducting zinc oxide dispersed in a silicone resin vehicle, the weight ratio of photoconducting zinc oxide to vehicle in said stratum being between 50/50 and 10, andy the vehicle being present in said substratum in proportions up to one part by weight vehicle to two and a half parts by weight semiconducting zinc oxide.

` 10. A print receiving element comprising' a paper sheet; a semiconducting substratum upon a surface of said sheet, said substratum comprising a major proportion of particles of semiconducting zinc oxide and a minor proportion of a caseinbinder, said binder being present -in said substratum in proportions up to one part by weight binder to two and a half parts by weight semiconducting `zinc oxide; and a photoconducting insulating stratum upon said substratum, said stratum comprising a major proportion of particles of photoconducting zinc oxide and a minor proportion of a silicone resin vehicle, said vehicle being present in said stratum in proportions between 50 and 90.weight parts photoconducting of particles of semiconducting zinc oxide and a minor proportion of a casein binder, said binder being present in said` substratum in proportion of one part by weight binder to two and a-half parts by weight semiconductyeo a `photoconducting insulating stratum comprising particles of photoconducting zinc oxide dispersed ina waterinsoluble, electrically-insulating, solid, film-forming vehicle upon a semiconducting substratum having a short --dark decay relative to the dark decay of said photoconsemiconducting zinc oxide,` stannic oxide, titanium dioxide, and basic lead carbonate,Y dispersed'in awater-soluj ble, solid, film-forming vehicle, the weight ratio of photof Yin said substratum in proportions upto' onel by lstratum upon said substratum, said stratum comprisinga major proportion lof particles of photoconducting zinc p ing zinc oxide; anda photoconducting insulating stratum upon said vsubstratum, said stratum comprising a major proportion of particles of photoconducting zinc oxide and a minor proportion of -a siliconevresin vehicle, saidy ve- -hicle being present in said stratum in proportions of weight parts photoconducting zinc oxide'and 40 weight parts vehicle, said photoconducting stratum having along 12. A-.print' receiving element comprising a paper sheet; a semiconducting substratum upon a surface of said 'sheet said substratum com risin '-a ma'orl ro ortion Y ducting stratum and comprismgpartlcles of a semiconp g l p p ducting fmaterial selected from'the group consisting of l' of particles of'scmiconducting zinc oxide' and a minor @proportion of a cellulose acetate binder, said binder being present in said'substratum -in proportions'upto one lpartp K v f by Weight binder to two anda half partsby w'eightsemi- I conducting zinc oxide; and a photoconducting insulating oxide and a minor proportion* oflapsilicone resin vehicle,

. 1ark decayrelative'to the dark decay offsaid substratum. 65

said vehicle being present in said stratum in proportions between 50 and 90 Weight parts'r photoconducting zine oxide and between 10 and 50 weight parts vehicle, said photoconducting stratum having along dark decay relai tive to the clark decay of said substratum.

13. A print receiving element comprising a paper sheet; a semicondncting subst'r'atum upon a surface of said sheet, said substratnrn comprising a. major proportion of particles of semiconductng Zinc oxide and a minor pro portion of-a celiulose acetate binder, said, binder being present in said substratum in proportions of one part by weight binder to two and a half parts by weight semiconducting zinc oxide; and a photoconducting insulating stratum upon said substratum, said stratum com prising a major proportion of particles of photoconducting Zinc oxide rund a minor proportion of a silicone resin vehicle, said vehiclebeing present in said stratum in proportions of 100 weight parts photoconducting zinc oxide and 40 weight parts vehicle,A said photoconducting stratum having a long dark decay relative to the dark decay of said substratum.

References Cited in the le of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct, 6, 1942 2,427,700 Atkinson Sept. 23, 1947 2,476,800 Blackburn July 19, 1949 '12 2,554,017 Daltonv v May 22,v 1951 2,613,301` Dubar etL al. Oct. 7, 19,52 2,662,957 Eisler Dec. 15, 1953 2,664,044 Dalton Dec. 29, 1953 2,735,784 `Greig et al. Feb. 21, 1956 2,761,849 Coler Sept. 4, 1956 2,798,959 IMoncri,e-Yeltes July 9, 1957 2,825,314 Walkup May 4, 1958 V2,862,815, Sugarrnan et al. Dec. 2, 1958 2,887,632 Dalton May 14, 1959 2,901,348 Dessauer et al. Aug. 25, 1959 FOREIGN PATENTS 449,303 Canada June 22, 1948 1,022,091 Germany Jan. 2, 1958 157,101 Australia June 16, 1954 OTHER REFERENCES Semi-Conductor Abstracts, vol. III, 11955 issue, pp. 85- 88.

The Condensed Chem. Diet., Reinhold (1956), p. 727 Methy Violet, and p. 322, @crystal violet.

Vartanian: Acta Physochemica U.R.S.S., vol. XXII, No. 2, pp. 20L-.22,4 (19.47).

Cochran: Batelle Tech. Review,rvol. 3, No. 7, pp. 71- 74.

Wainer: Photographic Engineering, vol. 3, No. 1, pp. 12.9.2 (1952). 

1. AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT INCLUDING A PHOTOCONDUCTING INSULATING STRATUM COMPRISING PARTICLES OF PHOTOCONDUCTING ZINC OXIDE DISPERSED IN AN ELECTRICALLYINSULATING, SOLID, FILM-FORMING VEHICLE IN CONTACT WITH A SEMICONDUCTING SUBSTRATUM COMPRISING PARTICLES OF A SEMICONDUCTING MATERIAL SELECTED FROM THE GROUP CONSISTING OF SEMICONDUCTING ZINC OXIDE, STANNIC OXIDE, TITANIUM DIOXIDE AND BASIC LEAD CARBONATE, IN A SOLID, FILM-FORMING VEHICLE, SAID PHOTOCONDUCTING STRATUM HAVING A LONG BARK DECAY RELATIVE TO THE DARK DECAY OF SAID SUBSTRATUM, THE WEIGHT RATIO OF PHOTOCONDUCTING ZINC OXIDE TO VEHICLE IN SAID STRATUM BEING BETWEEN 50/50 AND 90/10, AND THE VEHICLE BEING PRESENT IN SAID SUBSTRATUM IN PROPORTIONS UP TO ONE PART BY WEIGHT VEHICLE TO TWO AND A HALF PARTS BY WEIGHT SEMICONDUCTING MATERIAL 